1. Field of the Invention
The present invention relates to a transmission for a four wheel drive vehicle and more particularly to a power distribution apparatus of a twin shaft type transmission having an input shaft and a counter shaft.
2. Discussion of Prior Arts
Some of so-called twin shaft type transmissions (transmissions having an input shaft for inputting power from an engine and a counter shaft on which shift gears are mounted) for four wheel drive vehicles employ a bevel gear type center differential for distributing power between front and rear wheels, as disclosed in Japanese Patent Application Laid-open No. Toku-Kai-Hei 7-167257. According to the technology proposed in the disclosure, the center differential is disposed coaxially with the counter shaft and power after gearshifting is inputted from the counter shaft through a differential casing. A pair of pinions are rotatably mounted on the differential case so as to equally distribute power inputted to the differential casing between front and rear wheels through a pair of side gears meshing with the pinions, respectively. A front drive shaft is coaxially disposed in the counter shaft. The power transmission to the front drive shaft is directly performed through a front output shaft connected to one side gear. Further, a rear drive shaft is coaxially disposed with an input shaft from an engine and the power transmission to the rear drive shaft is performed through a rear output shaft connected to the other side gear and a pair of transfer gears.
Several technologies in which, in order to realize an unequal torque distribution between front and rear wheels and to enhance a cornering ability and a steering characteristic of a four wheel vehicle, a bevel gear type center differential is replaced with a planetary gear type center differential, have been proposed. For example, Japanese Patent Application Laid-open No Toku-Kai-Hei 5-112149 proposed by the applicant of the present invention discloses a technique in which, in an automatic transmission wherein an output shaft is disposed coaxially with an input shaft and a front drive shaft is arranged in parallel with these input and output shafts, a planetary gear type center differential is disposed coaxially with the output shaft and power transmitted from a first sun gear provided on the output shaft to a second sun gear through a second pinion is transmitted directly to a rear drive shaft and further power transmitted to a carrier through a pinion shaft supporting the first and second pinions is transmitted to a front drive shaft through a pair of reduction gears.
However, in case of the twin shaft type transmission as disclosed in Toku-Kai-Hei 7-167257, it is difficult to dispose a planetary gear type center differential on the same axis as the counter shaft in place of the bevel gear type center differential.
Since the front drive shaft is disposed inside of the counters haft, it is difficult to connect the reduction gear secured to the outer periphery of the carrier with the front drive shaft without incurring a large change of geometry and a complication of construction.
It is an object of the present invention to provide a twin shaft type transmission for a four wheel drive vehicle having an input shaft for inputting power of an engine and a counter shaft capable of disposing a planetary gear type center differential on the same axis as the counter shaft with a simple constitution.
To accomplish the object, a center differential accommodated in a twin shaft type transmission for a four wheel drive vehicle having an input shaft driveably connected with an engine, a hollow counter shaft arranged in parallel with the input shaft, shift gear trains provided between the input shaft and the counter shaft in a meshing relationship with each other, one of front and rear drive shafts coaxially provided in the counter shaft, the center differential comprises a hollow center differential input shaft connected coaxially with the counter shaft for inputting power to the center differential, a first sun gear mounted on the center differential input shaft, a second sun gear provided coaxially with the first sun gear for outputting power to the other of the front and rear drive shafts, a first pinion meshing with the first sun gear and provided around the center differential input shaft, a second pinion formed integrally with the first pinion and meshing with the second sun gear, a carrier rotatably holding the first and second pinions and enclosing the first and second sun gears and the first and second pinions therein for outputting power to one of the front and rear drive shafts, and a hub secured to the inside of the carrier and extending through a space between the first and second sun gears to the inside of the center differential input shaft and connected with one of the front and rear drive shaft.